As some molten metal alloys solidify after being poured into a container or mold, liquid inhomogeneities tend to develop in the partially solidified portion because of a difference in composition between the interdendritic liquid and the bulk liquid. In some alloys, particularly steels, the interdendritic liquid tends to become less dense than the bulk liquid, because of changes in temperature and composition, and tends to rise through the casting. In other alloys, the interdendritic liquid tends to become more dense than the bulk liquid and falls or sinks through the casting. In either case, as the interdentritic liquid percolates through the tree-like dendtritic system in the casting, local melting of previously solidified material leads to the development of larger channels through which large amounts of the less or more dense interdentritic liquids can rapidly flow. This condition not only can result in local inhomogenity, but, more importantly, can produce macrosegregation in the casting in the form of so-called "A" segregation in steel castings and severe localized "freckles" in electroslag (or vacuum arc) remelted ingots of steels and superalloys. This phenomena is discussed in more detail in R. Mehrabian et al, Interdentritic Fluid Flow and Macrosegregation; Influence of Gravity, Met. Trans., Vol. 1, 1209 (1970) and S.M. Copley et al, The Origin of Freckles in Unidirectionally Soldified Casting, Met. Trans. Vol. 1, 2193 (1970).
This type of macrosegregation occurs in ingots or castings having relative low rates of solidification and is more prevalent in large castings, for example, 10 ton castings of medium to high carbon steels. Under severe macrosegregation conditions, the carbon content in medium carbon steel ingots can vary from more than 1% in the top portion to less than 0.5% in the bottom portion. For applications where this variation in the composition is unacceptable, it is necessary to physically remove relatively large portions of the ingot, a costly and time consuming procedure. Some reduction in macrosegregation can be obtained by using a suitable combination of solutes which minimize density variations and thereby retard the development of segregation channels. However, such an approach ordinarily is effective only over limited composition ranges and the choices of solutes for this purpose may not be compatible with the specifications for the alloy.
It is known that the grain structure of a metal casting can be refined by agitating the casting mold during solidification, such as rotating or oscillating the mold in various manners. Examples of such methods are disclosed in U.S. Pat. Nos. 1,775,859 (Hultgren), 3,568,752 (Williams) and 3,614,976 (Bolling et al.), Japanese Patent No. 71/39597 and M. Stewart et al, Macrosegregation in Castings Rotated and Oscillated During Solidification, Met. Trans. Vol. 2, 169 (1971). In these methods the mold is rotated about a vertical axis or a horizontal axis, the mold is rotated in a manner to agitate or stir the bulk liquid, and/or the mold is not rotated during the entire period of solidification.
S. Kou et al., Macrosegregation in Rotated Remelted Ingots, Met. Trans. B. Vol. 9B, 711 (1978) discloses the use of centrifugal force, by rotating the mold about a vertical axis at a speed in the order of 76 rpm during solidification of the ingot, to reduce radial or horizontal macrosegregation.
Applicant is unaware of any prior publications disclosing the concept of rotating a mold or container about an axis inclined to the vertical at speeds substantially below those which produce centrifuging.